Presently, high density polyethylene (HDPE) with a bimodal MWD is produced with a tandem reactor process involving two or more polymerization reactors. In such a process, the catalyst is exposed to one of the reactors in the presence of very low levels of hydrogen as chain transfer agent, and then transferred into a second reactor with relatively large amounts of hydrogen. Under these conditions the first reactor product is the relatively high-molecular weight (HMW) component, while the second reactor produces the relatively low-molecular weight (LMW) component.
Titanium/zirconium-based bimetallic catalyst systems that produce broad/bimodal molecular weight distribution polyethylene resins in a single reactor have been described. The LMW component is produced by the Zr site, while the HMW component is produced by the Ti site. The relative productivity of the two sites determines the weight fraction of each of the HMW/LMW components in the final product. As with typical Ziegler/Natta catalysts, an aluminumalkyl cocatalyst is usually added to the polymerization reactor (either slurry or gas phase) in order to activate the catalyst to produce polyethylene.
The use of metallocene compounds of transition metals as catalysts for polymerization and copolymerization of ethylene is a relatively recent development. Metallocenes can be described by the empirical formula Cp.sub.m MA.sub.n B.sub.p. These compounds have been used to produce olefin polymers and copolymers, such as ethylene and propylene homopolymers, ethylene-butene and ethylene-hexene copolymers. Metallocene catalysts containing a second transition metal, such as titanium have been developed which produce bimodal molecular weight distribution products, having a high molecular weight component and a relatively lower molecular weight component. Such developments provide a commercial alternative to processes which require two or more reactors to produce bimodal molecular weight distribution product(s) with production of one of the molecular weight components in a first reactor and transfer of that component to a second reactor and completion of the polymerization with production of the other component of different molecular weight.
Metallocenes can be activated with alumoxanes, e.g. methylalumoxane (MAO). The class of alumoxanes comprises oligomeric linear and/or cyclic alkylalumoxanes represented by the formula: R--(Al(R)--O).sub.n --AlR.sub.2 for oligomeric, linear alumoxanes and (--Al(R)--O--).sub.m for oligomeric cyclic alumoxanes wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R is a C.sub.1 -C.sub.8 alkyl group and preferably methyl. Methylalumoxane is commonly produced by reacting trimethylaluminum with water or with hydrated inorganic salts, such as CuSO.sub.4 5H.sub.2 O or Al.sub.2 (SO.sub.4).sub.3 .multidot.5H.sub.2 O. Methylalumoxane can be also generated in situ in polymerization reactors by adding to the reactor trimethylaluminum and water or water-containing inorganic salts. MAO is a mixture of oligomers with a very wide distribution of molecular weights and usually with an average molecular weight of about 1200. MAO is typically kept in solution in toluene. While the MAO solutions remain liquid at fluidized bed reactor temperatures, the MAO itself is a solid at room temperature. Most of the experiments reported in the literature relating to methylalumoxane used as a cocatalyst with metallocene compounds are undertaken in a slurry or solution process, rather than in a gas phase fluidized bed reactor process. Reactor fouling results when MAO solutions are fed directly into the gas phase reactor in large enough quantities to provide liquid contact with the metallocene component of the catalyst. The fouling occurs because the MAO solution forms a liquid film on the interior walls of the reactor. The catalyst is activated when it comes into contact with this liquid film, and the activated catalyst reacts with ethylene to form a polymer coating which grows larger in size until the reactor is fouled. In addition, since substantially all of the activation takes place on the walls, the MAO is not uniformly distributed to the catalyst particles. The resulting non-homogeneous polymerization gives low catalyst activity and poor product properties.
Activation of the metallocene component of a bimetallic catalyst is via the alumoxane. A separate trialkylaluminum feed was used to activative the non-metallocene component of the bimetallic catalyst.